[scroll-animations-1] getCurrentTime() returns percentages on ScrollTimelines

Author: fantasai

scroll-animations-1/Overview.bs

@@ -913,6 +913,11 @@ spec: cssom-view-1; type: dfn;
 		2. Let |current time| 
 			be the value of [=this's=] {{AnimationTimeline/currentTime}} internal slot.
 
-			Create a [=new unit value=] from (|current time|, "ms")
+			If [=this=] is a {{ScrollTimeline}},
+			create a [=new unit value=] from (|current time|, "percent")
+			and return it.
+
+			Otherwise,
+			create a [=new unit value=] from (|current time|, "ms")
 			and return it.
 	</div>